1. Field of the Invention
This invention relates generally to breathing ventilators, and more particularly relates to a pneumatically driven, electronically controlled, ventilator system for providing breathing gas to a patient, and a system and method for flow triggering pressure support during patient-initiated spontaneous breathing. Pressure support is also known in the art by other names, e.g. inspiratory assist, pressure assist, or inspiratory pressure assist.
2. Prior Art
Breathing ventilator systems conventionally provide a breathing gas for either non-pressure supported breaths during inspiration at a pressure level typically no more than 2 cm. of water above or below the pressure baseline or pressure-supported breaths of breathing gas at a support pressure during inspiration as high as 70-100 cm. of water. Such breathing gas is often supplemented with a higher proportion of oxygen than is found in the ambient atmosphere. The respiration work performed by a patient on a ventilator may be divided into two major components: the work to initiate a breath and the work to sustain a breath. It is desirable to reduce the effort expended by the patient in each of these phases, since a high level of such effort can cause further damage to a weakened patient or be beyond the capabilities of small or disabled patients. As discussed below, a variety of strategies and systems have been developed to address these problems, but important issues still remain in the reduction of work demanded by ventilators to command and sustain a breath.
Ventilators presently known in the art are commanded to deliver inspiration support, or a specific flow of breathing gas during an inspiratory phase of breathing, based upon a "pressure trigger" as described below. With such a system, when a patient's spontaneous inspiratory effect withdraws a small volume of gas from the breathing-gas circuit, the corresponding drop in pressure in the closed ventilator circuit is monitored, and when a predetermined triggering pressure threshold is reached, a control mechanism causes the ventilator's pneumatic system to deliver breathing gas at the desired pressure or flow rate. This activation of the ventilator cycle by means of a patient-induced negative pressure may be termed "pressure triggering". A certain amount of lag time and associated negative pressure always occurs between the onset of inspiratory effort and the time that the gas pressure or flow reaches the patient's airway. This lag time (or delay) is generally referred to as a ventilator's response time, and commonly occupies a small but significant portion of a patient's total inspiration time.
Pressure triggering of inspiration support relies upon the transmission of pressure waves throughout the closed breathing-gas circuit. These pressure waves travel to the pressure sensor at the speed of sound in the gas, which is approximately 1 millisecond per foot. Although electronic processing of pressure-wave signals can occur very rapidly, due to factors inherent in ventilator design, patient-inspiration effort can typically continue for as long as 40 to 50 milliseconds without ventilator assistance. Under the conventional pressure-triggering ventilation schemes, the pressure drop, which a patient is required to create in a closed breathing-gas circuit in order to trigger a breath, can require a significant expenditure of energy by the patient. This imposed work on the patient can be detrimental in that respiratory muscles already fatigued by an operation or other patient condition may fatigue. In addition, this respiratory work may be beyond the capability of some patients, such as neonates, small children, or patients severely weakened by trauma or disease, resulting in the inability of the patient to rhythmically trigger the inspiratory support of the ventilator. If this process continues to worsen, the patient may experience failure or severe compromise of the ventilation process. Thus, the ventilator response time, plus the lag time associated with pressure triggering, can result in a significant expenditure of work by the patient in order to command a breath from the ventilator.
The signal to cycle on the ventilator to deliver pressure or volume support of patient breaths by monitoring flow in the patient's breathing-gas circuit or inside the ventilator has recently been accomplished in the context of a closed breathing-gas circuit. In such a system, a single flow sensor is typically positioned inside the ventilator to monitor the flow of gas that a patient withdraws from the closed system and trigger a pressure-or volume-based breath when the patient's inspiratory flow equals a certain predetermined level. However, such a closed-system, flow-based trigger is not an improvement over a closed-system pressure triggered arrangement, because all of the same delays and work required of the patient are present. In addition, a significant negative pressure drop is still required to start the breath, and there is no continuous flow to support the earliest phase of the breath. Therefore, the patient must overcome the substantial inertia of the breath triggering process.
In order to decrease the work of sustaining the flow of a breath after it has been initiated, thereby reducing the work required of our patient, breathing ventilator systems conventionally provide a breathing gas for non-pressure supported breaths during inspiration at a pressure level typically no more than 2 cm. of water above or below the pressure baseline. In pressure-supported systems, breaths of breathing gas are delivered at a pressure support level during inspiration as high as 70-100 cm. of water. These higher pressures are used to supplement patient effort, overcome airway resistance, and reduce the work of breathing for the patient. This use of a higher pressure support level can provide enhanced comfort for the patient, and may facilitate the weaning of the patient from the ventilator.
To circumvent or overcome the problems associated with breath triggering in the context of a closed ventilator circuit, a continuous flow system may be employed. To ensure that the patient receives a flow of breathing gas immediately upon initiation of an inspiratory effort and with the appropriate oxygen concentration, a flow regulator is positioned at the inlet of the breathing-gas circuit to deliver a constant gas flow in excess of the peak-flow demand expected from the patient. This "continuous flow" approach eliminates the ventilator's delay time and significantly reduces the negative pressure work associated with closed ventilator systems.
An advantage of the continuous gas flow (available in an open breathing-gas system) is that a patient's inspiratory effort results in an immediate flow of breathing gas into his/her trachea, without the delays and with less negative pressure work inherent in closed ventilator systems. Thus, it would be desirable to provide pressure support to a patent in an initially, open continuous-flow system rather than from a closed breathing gas system. It would also be desirable to provide a method and system for triggering breaths which can be made to be more sensitive than previous pressure-based strategies.
From the above, it is clear that it would be desirable to combine the advantages of flow sensing and triggering in a functionally open, breathing-gas circuit with pressure support for the purpose of enabling the ventilator to reduce significantly both the work of breathing during the earliest phase as well as the patient's breathing work during the later phases of the inspiratory effort. The present invention accomplishes these goals.